Traditionally the removal of mercaptans from various process materials and/or streams has been a substantial problem. The reasons for desiring this removal are well-known in the art and include: corrosion problems, burning problems, catalytic poisoning problems, undesired side reaction problems, offensive odor problems, etc.
The methods that have been proposed for the solution of this removal problem can be catagorized into those that seek the absolute removal of mercaptan compounds or any derivatives of these compounds from the carrier stream or material, and those that seek only to convert the mercaptans into a less harmful derivative with no attendant attempt at removal of these less harmful derivatives. Solutions of the former type are generally labeled as "extraction" processes. Solutions of the latter type are generally labeled as "sweetening" processes. Prominent among the extraction processes is a process which depends for its effectiveness on the fact that mercaptans are slightly acidic in nature and in the presence of a strong base tend to form salts--called mercaptides--which have a remarkably high preferential solubility in a basic solution. In this type of process, an extraction step is coupled with a regeneration step and an alkaline stream is continuously recirculated therebetween. In the extraction step, the alkaline stream is used to extract mercaptans from the hydrocarbon stream, and the resulting mercaptide rich alkaline stream is treated in the regeneration step to remove mercaptide compounds therefrom with continuous cycling of the alkaline stream between the extraction step and the regeneration step. The regeneration step is typically operated to produce disulfide compounds which are immiscible in the alkaline stream, and the major portion of which is typically separated therefrom in a settling step. In many cases, however, it is desired to remove substantially all disulfide compounds from the alkaline streams and complete separation of disulfide compounds from the alkaline stream in a settling step is not feasible because of the high dispersion of these compounds throughout the alkaline solution. Accordingly, the art has resorted to a number of sophisticated techniques in order to coalesce the disulfide compounds and effect their removal from the regenerated alkaline solution. One technique that has been utilized involves the use of a coalescing agent such as steel wool in order to spring disulfides from the regenerated alkaline solution. This technique, however, results in significant amounts of disulfides left in the alkaline solution. Another technique which has been widely utilized involves the use of one or more stages of a naphtha wash (see for example U.S. Pat. No. 3,574,093) in order to extract disulfide compounds from this alkaline solution. This technique has been widely utilized in the art, but it has several disadvantages: (1) it requires the availability of naphtha; (2) it requires large volumes of naphtha because of its low efficiency; (3) it requires a separate train of vessels and separators; and (4) it requires disposal of the contaminated naphtha.
As is well known to those skilled in the art, there are certain low boiling range hydrocarbon streams for which it is absolutely critical that the amount of sulfur compounds contained therein be held to a very low level. In many cases, this requirement is expressed as a limitation on the total amount of sulfur that can be tolerated in the treated stream--typically the requirement is for a sulfur content less than 50 wt. ppm calculated as elemental sulfur, and more frequently, the requirement is less than 10 wt. ppm sulfur. Accordingly, when a mercaptan extraction process of the type described above is designed to meet these stringent sulfur limitations, it is essential that the amount of disulfides contained in the regenerated alkaline solution be held to an extremely low level in order to avoid contamination of the extracted stream with disulfides. For example, in the sweetening of a hydrocarbon stream containing C.sub.3 and C.sub.4 hydrocarbons and about 750 wt. ppm mercaptan sulfur, an extraction process can easily be designed to produce a treated hydrocarbon distillate having about 5 wt. ppm mercaptan sulfur; however, without special treatment of the regenerated alkaline solution utilized, the total sulfur content of the treated hydrocarbon stream will be about 50 wt. parts per million because of disulfide compounds which are returned to the extraction step via the alkaline stream where they are transferred to the treated hydrocarbon stream.
The instant invention cures this problem by treating the disulfide containing alkaline solution in a reduction step whereby the disulfides are reduced back to mercaptans. Since the mercaptans are preferentially soluble in the alkaline phase, they are not transferred to the treated hydrocarbon stream. The reduction of disulfides to mercaptans is known in the art but is carried out for other purposes than that presented herein (See U.S. Pat. No. 4,072,584). Reduction of the disulfide can be accomplished by either hydrogenation of the disulfide with hydrogen over a hydrogenation catalyst or by electrochemical means wherein the disulfide is reduced at the cathode electrode of an electrochemical cell. Some of the broad advantages associated with this solution to the sulfur reentry problem are: (1) it eliminates the disposal problem and additional separation hardware required for naphtha washing; and (2) it minimizes the amount of mercaptides in the alkaline recycle stream charged to the extraction zone.